Your question is a good one and easily answered. Cost. The Imperial era is far more popular in terms of the general populace, so manufacturers make bulk Imperial gear, but generally of poorer quality. Conversely, Republic era gear tends to be of higher quality, limited numbers, and higher quality. Sadly, when the general populace think of the Legions, they think Gladiator (which didn't even get the armor right). Brass (which is incorrect but easier to get) Montefortino helmets usually cost between 300-500 dollars, while a steel (again, incorrect) Gallic G helmet runs about 180.